Conventionally, an aqueous ink as ink for ink jet-recording which is excellent in water resistance and light resistance of a recorded image on a recording material has been proposed. In the inkjet printing, ink droplets are ejected from a printer head for printing. A piezo method and a thermal method are mainly used as an ink ejection method for image recording applications. Particularly, in the thermal method, it is known that ink in a nozzle is boiled by a heater of a heating resistor element disposed in the printer head so as to eject the ink droplets, and thus due to a sudden change in the temperature inside the nozzle, phenomenon called kogation in which aggregates such as decompositions of a coloring material and multivalent metal salt are deposited on a surface of the heating resistor element occurs. In addition, it is also known that ink aggregates cause clogging of the head even in the piezo type inkjet method in which kogation does not occur.
One of causes of the aggregates generated in the ink is multivalent metal ions as impurities derived from pigments and water. For example, a large amount of calcium ions which easily react with anions and easily form a product which is sparingly soluble in water are contained in a pigment in some cases, which causes the deposits or the aggregates at the time of producing ink or using the ink, thereby adversely affecting ejectability and storage stability. Generally, the aqueous ink contains a surfactant and other additives in addition to the pigment and water. Examples of the frequently used surfactants include an anionic surfactant such as sulfate and phosphate of polyoxyethylene alkyl ether, and these anions are known to easily react with metal ions. Particularly, in the case where the anions react with the calcium ions, it is likely that a water-insoluble crystal such as calcium carbonate or calcium sulfate is generated. In addition, similarly, carbonate, sulfate, or phosphate which is generally used as a pH buffer or a pH regulator is also likely to generate the water-insoluble crystal. Even though the amount of these aggregates is extremely small, a droplet speed of the ink is decreased in long-time printing, which eventually causes the ink not to be ejected, and thus in the inkjet field where ejection reliability is regarded as important, it is strongly desired to reduce multivalent metal ions in ink.
Examples of a method for reducing the multivalent metal ions in the ink include a method for directly reducing the multivalent metal ions from the ink after preparation, and a method for reducing the multivalent metal ions contained in a pigment or an aqueous pigment dispersion which is a raw material of ink.
The aqueous pigment dispersion (referred to as a pigment paste in some cases) is obtained by dispersing a pigment into water with a surfactant or a resin having an ionic group at a high concentration, and is used as a typical raw material of ink by being mixed with water for dilution, a surfactant or other additives necessary for the ink properties. Accordingly, it can be expected to reduce aggregates generated in the obtained ink by reducing the multivalent metal ions in the aqueous pigment dispersion.
In addition, it is known that the multivalent metal ion may crosslink a resin adsorbed to a pigment surface, which contributes to the dispersion stability to thereby cause secondary aggregates of the pigment particles, and accordingly, the aqueous pigment dispersion in which the multivalent metal ions are reduced is desired from the viewpoint of the dispersion stability of the aqueous pigment dispersion.
As a method for preventing the generation of aggregates by reducing the multivalent metal or the multivalent metal ions in the aqueous ink or the aqueous pigment dispersion, for example, PTL 1 discloses that a particle having a chelate forming group or a fibrous resin is caused to be in contact with the aqueous ink or the aqueous pigment dispersion so as to remove the multivalent metal, and thereby it is possible to prevent the kogation or aggregates from being generated. The method of PTL 1 is an effective method for reducing the multivalent metal ions; however, a step of contacting or removing the particles having a chelate forming group or a fibrous resin was essential in the process of producing the aqueous ink.